After sending an email, it is practically out of your control. The new Confidential Mode of Gmail tries to give you back a little control by offering message expiration dates and making it difficult to forward the e-mail.
Confidential Mode, which is part of the new Gmail interface, works because technically it does not use standard email protocols to deliver the message. Confidential messages are hosted on Google servers, instead. This has its drawbacks: recipients who are not seeing the email in Gmail have to click on a link to open messages in a browser, for example. Still, it's a decent attempt to make email safe, and it's quite simple when all parties use Gmail.